Android 简易刮刮乐效果

PorterDuffXfermode设置的是两个图层交集区域的显示方式，dst是先画的图形，src时候画的图形。

详情参考：http://407827531.iteye.com/blog/1470519

public class XfermodeView extends View {

private Bitmap mBgBitmap, mFgBitmap;
private Paint mPaint;
private Canvas mCanvas;
private Path mPath;

public XfermodeView(Context context) {
super(context);
init();
}

public XfermodeView(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}

public XfermodeView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init();
}

private void init() {
mPaint = new Paint();
// 全透明
mPaint.setAlpha(0);
// 取两层绘制交集。显示下层。
mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
mPaint.setStyle(Paint.Style.STROKE);
// 笔触圆滑
mPaint.setStrokeJoin(Paint.Join.ROUND);
mPaint.setStrokeWidth(50);
mPaint.setStrokeCap(Paint.Cap.ROUND);
mPath = new Path();
mBgBitmap = BitmapFactory.decodeResource(getResources(),
R.drawable.test);
mFgBitmap = Bitmap.createBitmap(mBgBitmap.getWidth(),
mBgBitmap.getHeight(), Bitmap.Config.ARGB_8888);
mCanvas = new Canvas(mFgBitmap);
mCanvas.drawColor(Color.GRAY);
}

@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
mPath.reset();
mPath.moveTo(event.getX(), event.getY());
break;
case MotionEvent.ACTION_MOVE:
mPath.lineTo(event.getX(), event.getY());
break;
}
mCanvas.drawPath(mPath, mPaint);
invalidate();
return true;
}

@Override
protected void onDraw(Canvas canvas) {
canvas.drawBitmap(mBgBitmap, 0, 0, null);
canvas.drawBitmap(mFgBitmap, 0, 0, null);
}
}